The invention relates generally to the field of well logging. More particularly, the invention relates to improved techniques for well logging using nuclear magnetic resonance and methods for analyzing the nuclear magnetic measurements.
Oil well logging tools include nuclear magnetic resonance (NMR) instruments. NMR instruments can provide a wealth of information for formation evaluation that is not obtainable from other well logging measurements. Information provided by NMR measurements include the fractional volume of pore space, the fractional volume of mobile fluid filling the pore space, and the porosity of earth formations. General background of NMR well logging is described in U.S. Pat. No. 6,140,817A1, assigned to the assignee hereof.
The signals measured by nuclear magnetic resonance (NMR) logging tools typically arise from the selected nuclei present in the probed volume. Because hydrogen nuclei are the most abundant and easily detectable, most NMR logging tools are tuned to detect hydrogen resonance signals (form either water or hydrocarbons). These hydrogen nuclei have different dynamic properties (e.g., diffusion rate and rotation rate) that are dependent on their environments. The different dynamic properties of these nuclei manifest themselves in different nuclear spin relaxation times (i.e., spin-lattice relaxation time (T1) and spin-spin relaxation time (T2)) and diffusion constants. For example, hydrogen nuclei in viscous oils have relatively short relaxation times and low diffusivity, whereas hydrogen nuclei in light oils possess relatively long relaxation times and high diffusivity. Furthermore, the hydrogen nuclei in free water typically have longer relaxation times than those in bound water. Consequently, these differing NMR relaxation times can provide information on properties of the earth formations.
Most NMR logging tools measure the spin-spin relaxation times (T2) to derive the properties of the earth formations. T2 relaxation is often measured from a train of spin-echoes that are generated with a series of pulses such as the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence or some variants of this. The CPMG pulse sequence is well known in the art. See Meiboom, S., Gill, D., 1958, “Modified Spin Echo Method for Measuring Nuclear Relaxation Times,” Review of Scientific Instruments, 29, 688-91.
Although T2 measurements provide useful information for formation characterization, T2 information alone may not be sufficient for distinct characterization of earth formations, especially when different components in the formations have similar or overlapping T2 values. In order to differentiate different fluids (e.g., hydrocarbons versus connate water) with similar or overlapping T2 distributions, several differential methods have been proposed, such as the differential spectrum method (DSM) and time domain analysis (TDA). These methods takes advantages of different longitudinal relaxation times of different fluids. Two sets of measurements are made with different wait times (times for the spin to be polarized by the static magnetic field). One wait time is selected that one type of fluid (e.g., brine that has a longer longitudinal relaxation time T1) would not fully relax. As a result, the signal magnitudes from the long T1 fluid would be substantially reduced. Subtraction of these two sets of measurements would then produce a difference measurement which is comprised mostly of signals from the fluids with long T1 times. However, success of these approaches relies on the selection of proper wait times, which requires prior knowledge of the NMR properties of the fluids in the formation.
Recently, a magnetic resonance fluid characterization (MRF) method has been shown to provide more useful information. For a detailed discussion of the MRF method, see U.S. Pat. No. 6,229,308 B1 issued to Freedman and assigned to the assignee of the present invention. This patent is hereby incorporated by reference. When T2 distributions overlap, the MRF method distinguishes oil and water in porous media based on different molecular diffusion. For the same T2, oil and water have different diffusion constants. Therefore, contributions of different fluids to the measured T2 distributions can be separated by combining measurements with different sensitivity to diffusion. However, in order to use MRF, diffusion constants must be measurable. If T2 becomes comparable with or shorter than the decay due to diffusion, then the diffusion constant can no longer be determined, and the MRF method becomes impracticable. In other words, the MRF method is applicable only when T2 values are longer than a certain critical value.
While the MRF analysis has proved to be a powerful approach, it is desirable to have methods that can be used to analyze fluids with not only long T1 or T2, but also short T1 or T2, such as viscous fluids. Furthermore, it is desirable to have methods that can be used in logging tools with a low magnetic field gradient or a saddle point in the magnetic field.